The present invention relates to a filter circuit suitable for being integrated into a single IC chip.
The channel frequencies for low power-type cordless telephones in Japan are as follows.
______________________________________ up link channel (portable unit to base unit) 381 MHz band. down link channel (base unit to portable unit) 254 MHz band. channel interval 12.5 kHz. ______________________________________
FIG. 1 and FIG. 2 show typical receiving and transmitting circuits of a cordless telephone. Herein, lines indicated by symbols *1 and *2 in FIG. 1 and FIG. 2 should be connected to form full configuration of the circuits. These circuits meet the above standard and may be incorporated on a single chip of an IC (integrated circuit). This IC chip is designed to be usable in both of a base unit (base-side telephone) and a portable unit (portable telephone).
In FIG. 1 and FIG. 2, the portion surrounded by the dotted line is incorporated in a single chip. These figures show the IC1 used for the portable unit with the receiving and transmitting circuits respectively designated with reference numerals 10 and 40.
The receiving circuit 10 has a direct conversion-type structure employing a double super-heterodyning method. A down link channel FM signal Sr from the base unit is received by the antenna 2 and then the received signal is provided to first mixer circuits 12 and 22 for performing an orthogonal transformation to an I axis and a Q axis via a terminal T11, a high frequency amplifier 11, a terminal T12, a band-pass filter 3 that allows all of the down channels to band-pass and terminal T13.
Further, an oscillating circuit 30 generates a signal S30 with a stable reference frequency of, for example, 14.4 MHz. To attain this, a crystal oscillating circuit 6 is connected via the terminal T16.
The oscillated signal S30 from the oscillating circuit 30 is provided to a frequency dividing circuit 33, and to a frequency circuit 35 which divides an input frequency into 1/1152, thereby the signal S35 with a frequency of 12.5 kHz corresponding to the channel spacing is produced. This signal S35 is then provided to a PLL (phase locked loop) 31 as the reference frequency signal. The PLL 31 has a variable frequency dividing (not shown) to which a dividing ratio N31 is provided via a terminal T17 to be set therewithin.
The oscillated signal S31 with a frequency equal to the carrier frequency of the FM signal Sr is generated from the VCO311 of the PLL circuit 31.
The signal S31 is provided to the first mixer circuit 12 as a first local oscillated signal and provided to a phase-shifting circuit 32 where the signal S31 is phase-shifted by just .pi./2. The phase-shifted signal S32 is provided to the first mixer 22 as the first local oscillated signal.
Therefore, as shown in FIG. 3A, the received signal Sr has a signal component Sa within the lower side waveband and a signal component Sb within the upper side waveband. Further, taking
.omega.o: the carrier frequency (angular frequency) of the received signal Sr PA1 .omega.a: angular frequency of signal component Sa. .omega.a&lt;.omega.o PA1 Ea: amplitude of signal component Sa PA1 .omega.b: angular frequency of signal component Sb. .omega.b&lt;.omega.o PA1 Eb: amplitude of signal component Sb PA1 .DELTA..omega.a=.omega.o-.omega.a PA1 .DELTA..omega.b=.omega.b-.omega.o, PA1 then, PA1 Further, taking PA1 Therefore, taking PA1 S13=.alpha.a.multidot.cos .DELTA..omega.at+.alpha.b.multidot.cos .DELTA..omega.bt PA1 S23=-.alpha.a.multidot.sin .DELTA..omega.at+.alpha.b.multidot.sin .DELTA..omega.bt. PA1 S33=E2.multidot.sin .omega.st PA1 S34=E2.multidot.cos .omega.st PA1 wherein PA1 and taking: PA1 then, ##EQU2##
Sr=Sa+Sb PA2 Sa=Ea.multidot.sin .omega.at PA2 Sb=Eb.multidot.sin .omega.bt. PA2 E1: the amplitude of the first local oscillated signals S31 and S32, PA2 then PA2 S12, S22: output signals of the mixer circuits 12 and 22, PA2 then, ##EQU1## PA2 E2: amplitude of second local oscillating signals S33 and S34 PA2 .omega.s=2.pi.fs PA2 S14, S24: output signals from the mixers 14 and 24,
S31=E1.multidot.sin .omega.ot PA3 S32=E1.multidot.cos .omega.ot. PA3 (fs=approximately 55 kHz)
The signals S12 and S22 are provided to the low-pass filters 13 and 23 since the signal components with the angular frequencies .DELTA..omega.a and .DELTA..omega.b are necessary for an intermediate frequency (hereinafter referred to as IF) signal. The signal components with the angular frequencies .DELTA..omega.a and .DELTA..omega.b are provided as the first IF signals S13 and S23 from the low-pass filter 13. The signals S13 and S23 are expressed as follows:
In this case, as being apparent from the above equations and FIG. 3A, the signals S13 and S23 are base-band signals.
These signals S13 and S23 are provided to the second mixer circuits 14 and 24 respectively for I axis and Q axis of orthogonal transformation.
The oscillated signal S30 from the oscillating circuit 30 is provided to the frequency dividing circuit 33 and divided into a relatively low frequency signal S33. For example, the signal S33 is divided by 262 to a frequency of about 55 kHz. This signal S33 is provided to the second mixer circuit 14 as the second local oscillated signal and provided to the phase-shifting circuit 34. The phase-shifting circuit 34 phase-shifts this signal S33 by .pi./2. The phase-shifted signal S34 is provided to the mixer 24 as the second local oscillated signal.
Therefore, taking
The equations for signals S14 and S24 are transformed so that the value of the frequency difference does not become negative, ##EQU3##
The signals S14 and S24 are then provided to an adding circuit 15 and added to each other. The resultant added signal S15 expressed by the following equation is provided from the adding circuit 15: ##EQU4##
The added signal S15 has signal components as shown in FIG. 3B. Thus, the signal S15 is produced from the received signal Sr by replacing the carrier frequency (angular frequency) .omega.o with .omega.s in the frequency conversion. The signal S15 is the second IF signal having intermediate frequency fs.
This second IF signal S15 is provided to the FM demodulating circuit 18 via a band-pass filter 16 as an IF filter and limiter amplifier 17 and is demodulated to an audio signal or digital data for controlling a procedure such as protocol.
The demodulated audio signal is provided to a speaker 4 for a telephone receiver via low-pass filters 25 and 26 for removing unnecessary band components, an amplifier 27 and the signal line of terminal T14.
The demodulated digital data is provided to a microcomputer for system control (not shown in the drawings) via terminal T24, the low-pass filter 25 and the band-pass filter 28.
The above description is for the configuration and operation of the receiving circuit 10.
At the transmitting circuit 40, the audio signal is made to be an up channel FM signal. The frequency divided signal S35 from the frequency dividing circuit 35 is provided to a PLL 43 as a reference frequency signal and a signal defining a dividing ratio N43 is provided to the variable frequency dividing circuit (not shown in the drawings) of the PLL 43 via terminal T18.
During transmission of the audio signal, a control signal from the microcomputer is provided to the switching circuit 47 via the terminal T26 and the switching circuit 47 is connected in the state shown in the drawings. The audio signal from a microphone 5 of the telephone transmitter is then provided to a VCO 431 of the PLL 43 as an oscillating frequency control signal via terminal T15, an amplifier 41, a low-pass filter 42 for removing unnecessary band components and the switching circuit 47.
An FM signal St, which is frequency modulated by the audio signal from the low-pass filter 42, is thus provided from the VCO 431 in the up link channel paired with the down link channel in the receiving circuit 10.
This FM signal St is provided to the antenna 2 via a drive amplifier 44, an output amplifier 45 and a terminal T19 before being transmitted to the base unit (not shown in the drawings).
Further, when digital data for controlling protocol etc. is transmitted, the switching circuit 47 is connected to the opposite contact in a state reverse to that shown in FIG. 2 by the control signal from terminal T26. Digital data from the microcomputer is provided to the VCO 431 of the PLL 43 as an oscillating frequency control signal via terminal T25, the low-pass filter 46 for removing unnecessary band components and the switching circuit 47.
An FM signal St which is frequency modulated (MSK modulated) by the digital data is then provided from the VCO 431.
The FM signal St is provided to the antenna 2 via a drive amplifier 44, an output amplifier 45 and a terminal T19.
The above description is for the configuration and operation of the transmitting circuit 40.
With typical FM receivers, the intermediate frequency is as high as 10.7 MHz. An intermediate frequency filter should therefore be constructed from ceramic filters, so that they cannot be integrated into an IC.
However, with the aforementioned receiving circuit 10, the first IF signals S12 and S22 are base-band signals and the second intermediate frequency fs may be such a lower one as being 55 kHz. The filters 13, 23 and 16 may therefore be composed using active filters having resistors, capacitors and amplifiers. Further, the filters 25, 26 and 28 may also have the same active filter structure since these filters are used only for audio bands.
The receiving circuit 10 may therefore be integrated into an IC by excluding the band-pass filter 3 and the oscillating coil (not shown in the drawings). The same also applies to the transmitting circuit 40.
The whole of the receiving circuit 10 and transmitting circuit 40 shown in FIG. 1 and FIG. 2 may therefore be formed into a single monolithic IC.
In FIG. 1 and FIG. 2, this IC1 is used for a portable unit. However, this IC may also operate in a base unit by connecting terminals T14 and T15 to a 4 line/2 line conversion circuit of a base unit and replacing the frequency dividing ratios N31 with N43. Then, in this case, receiving of the up link channel signal is carried out by the receiving circuit 10 and transmission of the down link channel signal is carried out by the transmitting circuit 20.
The IC IC1 is therefore usable in a base unit. That is, the IC IC1 is also usable in both a portable unit and a base unit.
Of the filters 13, 23, 16, 25, 26, 28, 42 and 46 described above, the low-pass filters may be composed as is exemplified in FIG. 4 and the band-pass filters may be composed as is exemplified in FIG. 5.
The cut-off frequency fLOW of the low-pass filters is then expressed by EQU fLOW=1/2{2.pi.(C1.multidot.C2.multidot.R2.multidot.R3)**0.5},
and the central frequency fBPF and sharpness QBPF are expressed by EQU fBPF=1/{2.pi.(C1.multidot.C2.multidot.R1.multidot.R2)**0.5} EQU QBPF=(1/2)(R2/R1)**0.5}.
(X**0.5 indicates X to the power of 1/2, as is the case in the following).
Variations in the relative values of the capacitors and resistors in the IC can be kept sufficiently small but those in the absolute values do become large. The temperature coefficients of the resistors in the IC are also large.
This affects the cut-off frequencies (central frequencies) of the filters 13 to 46 to vary together with temperature.
Therefore, there is a need for an apparatus for resolving such problems.